1. Field of the Invention
This invention relates to the activated sludge process of wastewater treatment. The invention especially relates to methods and apparatuses for aerating mixed liquor in activated sludge processes which are conducted in closed loop reactors, such as oxidation ditches.
2. Review of the Prior Art
Many liquid waste treatment processes, commonly termed aerobic processes, supply bacteria and other microorganisms with dissolved oxygen for treating aqueous wastes such as municipal sewage, tannery wastes, dairy wastes, meat-processing wastes, and the like.
One such aerobic process is the activated sludge process, in which the microorganisms are concentrated as an activated sludge to be mixed with incoming wastewater, which supplies food for the organisms. The apparatuses in which the activated sludge process is conducted comprise an aeration basin (reactor basin) and a final clarifier (settling tank). The aeration basin serves as a culturing basin in which to generate the growth of bacteria, protozoa, and other types of microorganisms, so that they can consume the pollutants in the raw waste entering the basin by converting the pollutants into energy, carbon dioxide, water, and cells (biomass).
The activated sludge process is effective for controlling this conversion activity within the aeration basin, for settling the biomass within the clarifier, for overflowing the purified liquor or effluent from the clarifier to discharge, and for returning the settled biomass from the clarifier to the aeration basin. Thus, the activated sludge process is a suspended growth, aerobic, biological treatment process, using an aeration basin and a settling tank, which is capable of producing very pure, high quality effluent, as long as the biomass settles properly.
The microorganisms in circuit flow of mixed liquor within the endless channels of oxidation ditches can be supplied with sufficient oxygen within the first or aerobic portion of the cyclic flow that they can form nitrite and nitrate ions from ammonia, which is derived from broken-down proteins, and then can be sufficiently deprived of oxygen during a second portion of the cyclic flow that certain other species of microorganisms can use the nitrite ions and nitrate ions and/or sulfate ions as oxygen sources, provided that a carbon-supplying food source, such as methanol, incoming wastewater, absorbed organic matter, or biomass cell carbon, is available. This process causes nitrogen to be liberated from the mixed liquor as bubbles of gas and is termed denitrification.
However, if temperature, biomass concentration, food supply, oxygen supply, and the like should change so that the aerobic portion of the endless channel is increased in length at the expense of the anoxic portion thereof (within which denitrification occurs), denitrification can continue while the withdrawn mixed liquor is being clarified. The unfortunate result is that nitrogen bubbles can rise within the clarifier and seriously interfere with settling of the biomass to form sludge and clarified liquor.
Aerators used in activated sludge systems include bubble diffusers, mechanical surface aerators (both high speed and low speed), submerged turbine aerators, horizontal rotors, and gas-liquid jet aerators, such as eddy jets. The horizontal rotors may be fixed, adjustable in height, or floating and may be fitted with brushes, blades, cages, or discs. "Submerged turbine aerator" is a term used in the wastewater treatment industry to describe a mixing device which comprises one or more axial-flow propellers or radial-flow impellers, in which compressed air or high-purity oxygen is diffused through the sparge device, located underneath the lower propeller or impeller which is attached to a vertically disposed shaft. The propeller may or may not be located within a vertically disposed discharge or intake duct.
Only vertically mounted low-speed mechanical surface aerators, horizontally mounted rotor aerators with brushes, blades, cages, or discs, and jet aerators had been usable in oxidation ditches until the invention of the total barrier oxidation ditch, as disclosed in U.S. Pat. No. 4,532,038 and in U.S. Pat. No. 4,460,471, both of which describe means for mounting and utilizing submerged turbine aerators within a deep oxidation ditch having a barrier athwart the channel, and both of which are incorporated herein by reference, and the invention of the partial barrier oxidation ditch, as disclosed in U.S. Pat. No. 4,278,547.
Apparatuses and methods are described in commonly owned U.S. Pat. No. 4,460,471 and U.S. Pat. No. 4,260,486 that provide a barrier oxidation ditch equipped with a feed means for return sludge; a feed means for raw wastewater; a downpumping draft tube axial-flow pump; a mounting means for the pump; a mounting means for the draft tube; a gas dispersal means for diffusing gas into the liquor to form a gas-liquor mixture; a feed means for delivering compressed oxygen-containing gas to the gas dispersal means; a deep oxygen contact duct which is in flow connection with the draft tube of the pump, passes beneath the barrier means at a greater depth than the floor of the oxidation ditch channel and discharges on the downstream side of the intake pump; and a barrier means for: (a) forcing all mixed liquor through the intake pump on the upstream side of the barrier means, (b) preventing backmixing of aerated liquor to the pump intake, and (c) accumulating all aerated liquor on the downstream side of the barrier means. The deep oxygen contact duct is preferably J- or U-shaped.
The deep contact duct may contain throughout any selected portion of its length a gas diffuser, a gas bubble-splitting and mixing means, or an interfacial surface generator. The deep contact duct may also be extended in the direction of flow for a sufficient distance that substantially all of the aerobic activity of the ditch occurs within the duct and under a selected hydraulic overpressure that is greater than the pressure corresponding to the depth of the channel.
The term "oxidation ditch" is currently used for relatively shallow oval-shaped basins in which mixed liquor is continuously circulated by horizontally mounted surface aerators, such as cage rotors and disc rotors, and other terms, such as continuous looped channel and endless channel, are currently used for basins in which the mixed liquor is continuously circulated by surface aerators through a plurality of side-by-side channel portions which have adjoining walls and square or semi-cylindrical ends providing connections between adjacent channel portions. However, the term "oxidation ditch" is employed herein as a general term encompassing both shallow and deep basins, whether circular, oval, or looped in any endless continuous loop or spiral configuration.
In such a closed-circuit oxidation ditch, this invention comprises at least one flow-control apparatus which provides repetitive aerobic treatment to all of the mixed liquor within the channel of the oxidation ditch. The flow-control apparatus of a total barrier oxidation ditch comprises a barrier which is sealably attached to the bottom and sides of the oxidation ditch and divides the mixed liquor into upstream liquor within an intake portion of the channel and downstream liquor within a discharge portion of the channel.
In a partial barrier oxidation ditch, the barrier may comprise an opening or gateway therein which may be continguous with one or both sides or the bottom and which may be fixed or adjustable in size. In a non-barriered oxidation ditch, there is no barrier but a water barrier which prevents backmixing. These three barrier embodiments are hereinafter designated total, partial, or non-barriered.
The flow-control apparatus also comprises at least one axial-flow pump which operates within an inlet means, such as an inlet opening formed of concrete, or a draft tube and is disposed to receive the upstream liquor and pump it downwardly and further comprises a sparging device, generally mounted within the inlet means, for diffusing an oxygen-containing gas in the mixed liquor. Each pump/aerator is mounted by being attached to the aerator support bridge and comprises a motor, a speed-reduction means, a pump shaft, an axial-flow pump impeller attached to the lower end of the shaft, at least one ring sparge, and a downdraft tube surrounding the impeller and the sparge ring.
The deep oxygen contact duct is preferably connected to the downdraft tube, leads downwardly to any desired depth, curves in a downstream direction, and leads upwardly to a discharge point within the discharge portion of the channel.
It is pertinent to note that a conventional circuit-flow oxidation ditch of the prior art operates as a complete mix system except that its D.O. gradient is characteristically plug flow. Circulation of the entire basin contents during each cycle, while admixing the mixed liquor with the relatively minor stream of inflowing wastewater, ensures such complete-mix conditions.
Circular and elongated oxidation ditches in which the mixed liquor is aerated and propelled by banks of jet ejectors are described in U.S. Pat. No. 3,846,292 of Lecompte, Jr. (1974); No. 3,897,000 of Mandt (1975); and 4,199,452 of Mandt (1980). These jet ejectors, such as directional-mix jet aerators manufactured by Pentech Division of Clevepak Industries, Inc., Cedar Falls, Iowa, can be used in deep oxidation ditches, having a depth such as 20 feet, and can thereby have high rates of oxygen transfer compared to surface aeration methods while using diffused or subsurface aeration. However, flow above the jet aerators is induced flow, not pumped flow, so that it is unaerated and later blends with the highly aerated liquor which is ejected from the jets. Backmixing of aerated liquor readily occurs as eddies develop above the jets. Since multiple jet headers must act as booster pumps around the entire channel of a typical oxidation ditch in order to obtain adequate oxygen transfer, the entire channel must be constructed with considerable depth (typically, 10 feet or greater) in order to provide relatively high oxygen transfer efficiencies.
An oxidation ditch which includes a bank of venturi-type ejectors is described in U.S. Pat. No. 3,990,974 of Sullins (1976). A dissolved oxygen sensor controls pumping of liquor through the ejectors and thereby the quantity of air sucked from above the liquor level into the throats of the ejectors. Banks of vertical settler tubes are also disposed at one end of the oxidation ditch to effect settling of particles after impingement upon the inside surfaces of the tubes.
An oxidation ditch having a transversely disposed barrier across the channel is described in Hungarian Pat. No. 166,160 (1976) and Austrian Pat. No. 339,224 (1977), the barrier being traversed by a discharge duct containing a smaller duct within which an axial-flow impeller is disposed, whereby the smaller duct discharges within the larger one and functions as an upwardly discharging jet ejector because air bubbles are discharged from the blades of the impeller.
As of 1975, there were more than 500 municipal oxidation ditch installations of the horizontal rotor type in the United States and 90 in Canada, and there were 154 Carrousel installations in the world, according to "A Comparison of Oxidation Ditch Plants to Competing Processes for Secondary and Advanced Treatment of Municipal Wastes", by W. F. Ettlich, EPA-600/2-78-051, March 1978, National Technical Information Service, Springfield, Va., 22161. In this publication, these oxidation ditch plants are stated to provide flexibility in operation, a stable sludge, and performance above the average of all other competing secondary processes. Oxidation ditch plants were also found to be very competitive in operation and maintenance cost and to provide nitrogen removal at no additional cost.
However, these prior art oxidation ditches have many design and operational problems. Except for the jet ejector types, the pump/aerator devices of all prior art oxidation ditches are surface aerators. These devices produce spray and mist which create slippery walkways because of algae growth in summer and freezing in winter. They also cause excessive ice formation on the aeration equipment in the winter. Enhancing the surface area of liquor exposed to cold air by surface aeration further causes a loss of heat from the system and a reduction in reaction rates.
It is conventional practice in prior art oxidation ditches that their pump/aerators furiously aerate a portion of their liquor, while allowing the remainder to flow past untouched, and then the aerated and unaerated portions of the mixed liquor blend somewhere downstream of the pump/aerators to produce the desired dissolved oxygen content. From a hydraulic viewpoint, this practice can be termed "booster pumping", because the pump merely accelerates or adds energy to the mass of liquor flowing past the pump.
Such booster pumping seems to have developed because designers of prior art oxidation ditches have apparently believed that the kinetic energy in the induced-flow liquor is an asset that should not be interfered with. They have accordingly designed their ditches for booster pumping with single devices that combine the functions of pumping and aerating, whereby the momentum of the flowing liquor is merely augmented with each circuit-flow movement past the pump/aerator. Because the pumping function requires a relatively small input of energy, the principal capability of these devices is the aerating function. However, pumping and aerating functions cannot be utilized independently.
In consequence, a multi-component price has had to be paid for this value judgment as to the importance of kinetic energy. These price components include:
a. Heterogeneous aeration occurs when unaerated induced-flow liquor blends with highly aerated liquor to produce a blended downstream liquor having a desired average dissolved-oxygen content. Such heterogeneous aeration requires more energy than homogeneous aeration to the same dissolved-oxygen content because the same quantity of oxygen must be directly transferred, at a lower transfer rate, to a smaller volume of mixed liquor circulating in the oxidation ditch. PA0 b. Inflexibility of operation occurs because the aerating and pumping functions of the pump/aerator are performed simultaneously by the same prior art device, whereby changing the submergence or the rotational speed of a pump/aerator simultaneously changes both the oxygen transfer rate and the oxidation ditch circulation velocity when it may be desirable to reduce oxygen transfer rate but not reduce oxidation ditch circulation velocity, for example. PA0 C=initial oxygen concentration, PA0 t=time PA0 C.sub.s =saturation concentration of oxygen at the given temperature, and PA0 K=the overall gas mass transfer coefficient (time.sup.-1); it is a function of the resistance of the films and the area of liquid-gas interface per unit volume of liquid.
c. Aeration efficiency cannot be improved by increasing the driving force of oxygen transfer into the mixed liquor by using higher hydrostatic pressures selectively in the oxygen transfer zone of the oxidation ditch.
Balancing the disadvantages of heterogeneous aeration is the benefit imparted by preserving at least a portion of the momentum in the translationally moving mixed liquor by providing a gateway in the barrier. Such a partial barrier oxidation ditch is disclosed in U.S. Pat. No. 4,278,547 which is incorporated herein by reference.
The inability to prevent backpumping of liquor from the discharge side toward the intake side of the aerator imposes some additional energy demands upon the system, while the inability to prevent backmixing of aerated mixed liquor with unaerated liquor is a much more serious cause of energy wastage. Backmixing reduces the amount of oxygen transferred into a given volume of mixed liquor circulating past or through a given aerator per unit of time. Additional energy is required to transfer a certain amount of oxygen per unit time or to attain a desired D.O. content when backmixing occurs because the necessary driving force for transferring oxygen increases non-linearly as the dissolved-oxygen content increases, according to the equation: EQU dC/dt=K(C.sub.s -C)
where, at a given temperature of oxygen transfer:
As may be appreciated from FIG. 1, the rate of oxygen transfer, from bubbles of an oxygen-containing gas, such as air, to water, is a tangent, dC, to the solubility curve plotted from this equation (for initially deaerated water at 4.degree. C. and an atmospheric pressure of 14.54 psi) at any time, t. If unaerated water is being aerated, the initial slope is quite steep, such as line A in FIG. 1 (equalling 4.0 in the units as shown). If backmixing occurs so that a mixture of aerated and unaerated water is aerated, the slope is much shallower, such as line B in FIG. 1 (equalling 1.1). If heavily aerated water reaches the aeration device, the slope can be very shallow, such as line C in FIG. 1 (equalling 0.53). It should, therefore, be quite clear that, with a given input of energy, backmixing will cause a considerably smaller quantity of oxygen to be transferred into a given circulation flow of water, as compared to the situation for aeration of unaerated water. Designers of prior art oxidation ditches, complete mix systems, and plug flow systems appear to have ignored the high cost of backmixing and even its very existence.
The inability of prior art oxidation ditches to prevent heterogeneous aeration, which occurs when highly aerated mixed liquor is blended with unaerated liquor, is also believed to be important. As can be appreciated by a glance at FIG. 1, when a quantity of aerated liquor having an oxygen concentration of 13.6 mg/l and an oxygen transfer rate C, after cumulative aerating for 10 minutes, is blended with an equal quantity having an oxygen concentration of zero and an oxygen transfer rate A, at zero aerating time, to produce a mixture having an oxygen content of 6.8 mg/l and an average cumulative oxygen transfer rate D (equalling 1.9), an average cumulative time of 5.0 minutes is spent on the entire blend, i.e., more energy is expended than would be required for homogeneously aerating up to slope D at cumulative time equalling 2.7 minutes. Expressed in other terms, aerating the entire quantity at a starting D.O. of 0.0 mg 0.sub.2 /1 for 5.0 minutes imparts 9.9 mg/l to the liquid, equivalent to line B. The difference between 9.9 and 6.8 represents a significant energy wastage when using the curve shown in FIG. 1 which is characteristic of one type of pump/aerator, as an example.
The combined effect of backmixing of aerated liquor to the aerator and of heterogeneous aeration, which occurs when unaerated liquor flows past the aerator and blends with aerated liquor somewhere downstream, is to force the final aeration time, t, to shift a significant distance along the aeration curve for the liquor, as illustrated by moving from the relatively steep tangent D to the shallower tangent B or even tangent C in FIG. 1.
All prior art oxidation ditches also lack a means to sparge or diffuse large quantities of air into the mixed liquor within the confined space of a deep oxygen contact duct to form an air-liquor mixture and thereby significantly increase oxygen transfer efficiency by raising oxygen dissolution pressure and oxygen solubility and by concentrating a high mixing power per unit volume into this confined space. The sparge tubes disclosed in U.S. Pat. No. 4,260,486 do provide a considerable amount of air, but they lack a means for concentrating a large number of high-volume air-diffusing devices within the ducts. Although jet aerators are adapted to form air-liquor mixtures in an exceptionally efficient manner, there is no known mounting means for installing and operating a plurality of them within the confined space of a deep oxygen contact duct.
Another serious problem is that a deep oxygen contact duct may be up to ten feet in diameter. The difference in hydrostatic pressure between the upper and lower jet aerators in a vertically disposed assembly thereof within a duct of such a size would accordingly cause at least a portion of these jet aerators to become inoperative. A means and a method for providing uniform pressures for each jet aerator are accordingly needed if jet aerators are to be seriously considered for such large contact ducts.
Furthermore, all ejection devices for air and liquids are subject to plugging, and this handicap would be especially likely if return sludge should be used as the liquid to be ejected. A means and a method for selectively cleaning the jet nozzles would therefore also be needed, regardless of the depth of installation of the jet aerators.